Smelting is one approach for recovering a metal, such as copper, from a metal-bearing sulfide material. However, due to the high cost of smelting, the copper sulfide minerals in an ore body typically are first concentrated by flotation techniques to provide a smaller volume for smelting. The concentrate is then shipped to a smelter, which processes the concentrate pyrometallurgically at high temperatures to form a crude copper product that is subsequently refined to a highly pure metal.
The recovery of copper from copper sulfide concentrates using pressure leaching has proven to be a potentially economically attractive alternative to smelting. Pressure leaching operations generally produce less fugitive emissions than smelting operations, and thus, environmental benefits may be realized. Further, pressure leaching circuits may be more cost-effectively constructed on-site at a concentrator, eliminating the expense associated with concentrate transportation that smelting operations may require. Further, any by-product acid produced in the pressure leaching circuit may be able to be used in adjacent heap leaching operations, thus offsetting some of the costs associated with purchased acid.
The mechanism by which pressure leaching processes effectuate the release of copper from sulfide mineral matrices, such as chalcopyrite, is generally dependent on temperature, oxygen availability, and process chemistry. In high temperature pressure leaching processes, that is, pressure leaching processes operating above about 215° C., the dominant oxidation reaction is believed to be as follows:4CuFeS2+4H2O+17O2→4CuSO4+2Fe2O3+4H2SO4  (1)If insufficient oxygen is present in the process vessel, the conversion of iron to hematite (Fe2O3) generally will be incomplete, resulting in the formation of ferrous sulfate, an undesirable reaction by-product.
In high temperature pressure leaching, the sulfur contained in the metal-bearing material (e.g., concentrate) typically is converted to sulfate. In connection with such pressure leaching processing operations, the copper typically is recovered from the resulting solution by solvent extraction and electrowinning techniques to provide a cathode copper product of high purity.
In solvent extraction (or solution extraction or liquid ion exchange, as it is sometimes called), the pregnant leach solution typically is mixed with an organic solvent (i.e., an extractant), which selectively removes the copper from the pregnant leach solution. The copper-loaded extractant is then mixed with an aqueous acid solution, which strips the copper from the extractant, producing a solution stream suitable for electrowinning. This resultant solution stream is highly concentrated in copper and relatively pure, and typically is processed into high quality cathode copper in an electrowinning circuit.
In general, electrowinning of copper consists of the electrolytic deposition (sometimes called “plating”) of copper onto a cathode and the evolution of oxygen at an anode. In a simple design of an exemplary electrowinning unit, a set of cathodes and anodes are set in a reaction chamber containing the copper-containing electrolyte. When the unit is energized, copper ions are reduced onto the cathode (i.e., plated). Plating of copper typically occurs on copper starter sheets or stainless steel blanks. Anodes are quasi-inert in the electrolyte and provide a surface for oxygen evolution. The copper plates produced by the electrowinning unit can be in excess of 99.99 percent pure.
Purification of copper from the pregnant leach solution by solvent extraction has proven to be a successful means of providing a concentrated copper solution suitable for electrowinning of highly pure copper metal. However, prior art teachings suggest the importance of ensuring that the acid concentration of the pregnant leach solution is appropriately controlled, often through neutralization, such as through the use of lime or acid-consuming ore.
Still others have recognized that the use of lime to neutralize the acid in the solution not only increases operating costs due to lime consumption but also may result in the formation of a low pulp density slurry, thus tending to make it more difficult to recover the copper from that slurry. In response, Placer Dome, Inc., of Vancouver, British Columbia, Canada, has proposed in, for example, U.S. Pat. Nos. 5,698,170 and 5,895,633 methods to recover copper from copper-containing materials, especially copper from copper sulfides such as chalcopyrite, wherein a copper-containing solution containing an acid is contacted, that is, diluted, with an aqueous diluent containing no more than about 5 grams/liter acid to yield a diluted copper-containing solution having an acid concentration ranging from about 2 to about 8 grams/liter prior to the step of solvent extracting the copper from the diluted copper-containing solution. In their patents, Placer Dome requires the significant use of a diluting solution to lower acid levels in the copper-containing solution sufficiently for favorable equilibrium conditions during solvent extraction, which technique Placer Dome suggests significantly reduces copper losses relative to many existing processes in which neutralization of the acid in the solution before solvent extraction is employed.
To achieve these results, Placer Dome teaches that the desired acid concentration ranges can be obtained when a sufficient amount of diluting solution is contacted with the copper-containing solution to yield the diluted copper-containing solution. Specifically, Placer Dome teaches that the ratio of the volume of copper-containing solution to the volume of diluting solution must range from about 1:10 to about 1:500. In this manner, the acid generated in pressure leaching is neutralized after, and preferably not before, solvent extraction and electrowinning.
While Placer Dome's patented process is usable in many situations, in cases where it is desirable to reduce operating costs and/or the metal-bearing ore at a particular site does not warrant such conditions, it would be desirable to obtain high metal recovery in processes where such dilution is not required.